We propose to use the two-microelectrode voltage clamp technique to examine the relations between membrane voltage, membrane ionic currents, and contraction of sheep cardiac Purkinje fibers. The principal goal is analysis of the slow inward current (Isi) and its relation to contraction. Isi appears to be directly linked to contraction, and slowly conducted responses arising from the Isi system may cause arrhythmias. Isi has not been measured reliably in Purkinje fibers because other currents distort it or mask it completely. The largest single competing current appears to be eliminated if fibers are bathed in a low chloride solution containing 0.5 mM 4-Aminopyridine, and a deflection of the net current which resembles the presumed time course of Isi is seen throughout the physiological voltage range. We plan to examine if Isi is indeed revealed by these treatments, and to analyze the kinetics of Isi. We will examine the relation of isi to contraction, and we hope to determine if such phenomena as aftercontractions and transient inward currents, which follow depolarizations, are related to the preceding calcium entry via Isi. These last mentioned studies should provide information about the cellular basis of triggered activity in the heart. The whole project should provide valuable information about how cardiac electrical activity and contraction are coupled.